I. Field of the Invention
The instant invention relates generally to variable displacement axial piston type fluid energy translating devices and more specifically to the control devices therefor.
II. Description of the Prior Art
A common type of axial piston fluid energy translating device is a pump or motor which includes a housing having a rotatably mounted barrel with a plurality of circumferentially spaced cylinder bores. A port plate is interposed between the barrel and the inlet and working ports of the device to alternately connect each cylinder with the inlet and working ports of the device as the barrel is rotated. Within each bore is a piston which is connected by shoes to a pivotable rocker cam assembly which reciprocates the pistons to pump fluid as the barrel is rotated.
In one form of variable displacement axial piston pump, the rocker cam assembly is pivoted about an axis perpendicular to the axis of rotation of the barrel to vary the inclination of the thrust plate assembly. This changes the stroke of the pistons and consequently changes the displacement of the pump. In such pumps, a control device is provided to vary the inclination of the rocker cam.
In U.S. Letters Pat. No 3,739,691 to Bobier, a variable displacement axial piston pump is shown with a rocker cam assembly on a pivotable yoke. As the yoke pivots, the rocker cam assembly is pivoted with respect to the cylinder barrel to change the stroke of the pistons. An L-shaped arm on the yoke has a slot which engages a connecting pin. This pin is connected to a displacement control device.
In one embodiment shown in the Bobier patent, the displacement control device is a piston mounted in a housing bore and positioned by a thumbscrew.
In another embodiment shown in the Bobier patent, the yoke has a pair of transverse control arms each engaged by a pair of opposed movable pistons.
U.S. Letters Pat. No. 2,945,449 to Le Febvre et al shows a rocker cam assembly on a tilt block having a convex back which rides upon opposed pairs of rollers.
The displacement control device shown in the Le Febvre patent is a spring centered hydraulic piston which is connected to the rocker cam by a mechanical linkage. The piston is operated by a hydraulic control valve which includes a follow-up mechanism. Another prior art displacement control device is shown in U.S. Pat. No. 3,302,585.
In such prior art control mechanisms, the displacement control device is connected to the rocker cam by a mechanical linkage. A disadvantage of such mechanisms is the inherent tolerances in mechanical linkage which may cause free play and may make precise positioning of the rocker cam difficult. Further, the amount of free play may increase as the linkage wears.